Whenever I am sprint jumping, and I press the "4" key on my keyboard, it doesn't work. All other keys work, and the 4 key does still work when I am standing still, walking, sprinting, or jumping. I have mentioned it to some of my friends and they have said that it happens to them too. You can still scroll to the 4 slot, the hotkey just doesn't work.
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This is a hardware limitation of your keyboard. See https://en.wikipedia.org/wiki/Rollover_(key)